1. Field of the Invention
The present invention refers to processes for obtaining a gallium-containing nitride crystal by an ammonobasic method as well as the gallium-containing nitride crystal itself. Furthermore, an apparatus for conducting the various methods is disclosed.
2. Discussion of the Related Art
Optoelectronic devices based on nitrides are usually manufactured on sapphire or silicon carbide substrates that differ from the deposited nitride layers (so-called heteroepitaxy). In the most often used Metallo-Organic Chemical Vapor Deposition (MOCVD) method, the deposition of GaN is performed from ammonia and organometallic compounds in the gas phase, and the growth rates achieved make it impossible to provide a bulk layer. The application of a buffer layer reduces the dislocation density, but not more than to approx. 108/cm2. Another method has also been proposed for obtaining bulk monocrystalline gallium nitride. This method consists of an epitaxial deposition employing halides in a vapor phase and is called Halide vapor Phase Epitaxy (HVPE) [xe2x80x9cOptical patterning of GaN filmsxe2x80x9d M. K. Kelly, O. Ambacher, Appl. Phys. Lett. 69 (12) (1996) and xe2x80x9cFabrication of thin-film InGaN light-emitting diode membranesxe2x80x9d W. S. Wrong, T. Sands, Appl. Phys. Lett. 75 (10) (1999)]. This method allows for the preparation of GaN substrates having a 2-inch diameter.
However, their quality is not sufficient for laser diodes, because the dislocation density continues to be approx. 107 to approx. 109/cm2. Recently, the method of Epitaxial Lateral Overgrowth (ELOG) has been used for reducing the dislocation density. In this method the GaN layer is first grown on a sapphire substrate and then a layer with SiO2 is deposited on it in the form of strips or a lattice. On the thus prepared substrate, in turn, the lateral growth of GaN may be carried out leading to a dislocation density of approx. 107/cm2.
The growth of bulk crystals of gallium nitride and other metals of group XIII (IUPAC, 1989) is extremely difficult. Standard methods of crystallization from melt and sublimation methods are not applicable because of the decomposition of the nitrides into metals and N2. In the High Nitrogen Pressure (HNP) method [xe2x80x9cProspects for high-pressure crystal growth of III-V nitridesxe2x80x9d S. Porowski et al., Inst. Phys. Conf. Series, 137, 369 (1998)] this decomposition is inhibited by the use of nitrogen under the high pressure. The growth of crystals is carried out in molten gallium, i.e. in the liquid phase, resulting in the production of GaN platelets about 10 mm in size. Sufficient solubility of nitrogen in gallium requires temperatures of about 1500xc2x0 C. and nitrogen pressures in the order of 15 kbar.
The use of supercritical ammonia has been proposed to lower the temperature and decrease the pressure during the growth process of nitrides. Peters has described the ammonothermal synthesis of aluminium nitride [J. Cryst. Growth 104, 411-418 (1990)]. R. Dwilinski et al. have shown, in particular, that it is possible to obtain a fine-crystalline gallium nitride by a synthesis from gallium and ammonia, provided that the latter contains alkali metal amides (KNH2 or LiNH2). The processes were conducted at temperatures of up to 550xc2x0 C. and under a pressure of 5 kbar, yielding crystals about 5 xcexcm in size [xe2x80x9cAMMONO method of BN, AlN, and GaN synthesis and crystal growthxe2x80x9d, Proc. EGW-3, Warsaw, Jun. 22-24, 1998, MRS Internet Journal of Nitride Semiconductor Research, http://nsr.mij.mrs.org/3/25]. Another supercritical ammonia method, where a fine-crystalline GaN is used as a feedstock together with a mineralizer consisting of an amide (KNH2) and a halide (KI) also provided for recrystallization of gallium nitride [xe2x80x9cCrystal growth of gallium nitride in supercritical ammoniaxe2x80x9d J. W. Kolis et al., J. Cryst. Growth 222, 431-434 (2001)]. The recrystallization process conducted at 400xc2x0 C. and 3.4 kbar resulted in GaN crystals about 0.5 mm in size. A similar method has also been described in Mat. Res. Soc. Symp. Proc. Vol. 495, 367-372 (1998) by J. W. Kolis et al. However, using these supercritical ammonia processes, no production of bulk monocrystalline was achieved because no chemical transport processes were observed in the supercritical solution, in particular no growth on seeds was conducted.
Therefore, it is an object of the present invention to provide an improved method of preparing a gallium-containing nitride crystal.
The lifetime of optical semiconductor devices depends primarily on the crystalline quality of the optically active layers, and especially on the surface dislocation density. In case of GaN based laser diodes, it is beneficial to lower the dislocation density in the GaN substrate layer to less than 106/cm2, and this has been extremely difficult to achieve using the methods known so far. Therefore, a further object of the invention is to provide gallium-containing nitride crystals having a quality suitable for use as substrates for optoelectronics.
The above objects are achieved by the subject matter recited in the appended claims. In particular, in one embodiment the present invention refers to a process for obtaining a gallium-containing nitride crystal, comprising the steps of:
providing a gallium-containing feedstock, an alkali metal-containing component, at least one crystallization seed and a nitrogen-containing solvent in at least one container;
bringing the nitrogen-containing solvent into a supercritical state;
(iii) at least partially dissolving the gallium-containing feedstock at a first temperature and at a first pressure; and
(iv) crystallizing gallium-containing nitride on the crystallization seed at a second temperature and at a second pressure while the nitrogen-containing solvent is in the supercritical state;
wherein at least one of the following criteria is fulfilled:
(a) the second temperature is higher than the first temperature; and
(b) the second pressure is lower than the first pressure.
In a second embodiment a process for preparing a gallium-containing nitride crystal is described which comprises the steps of:
(i) providing a gallium-containing feedstock comprising at least two different components, an alkali metal-containing component, at least one crystallization seed and a nitrogen-containing solvent in a container having a dissolution zone and a crystallization zone, whereby the gallium-containing feedstock is provided in the dissolution zone and the at least one crystallization seed is provided in the crystallization zone;
(ii) subsequently bringing the nitrogen-containing solvent into a supercritical state;
(iii) subsequently partially dissolving the gallium-containing feedstock at a dissolution temperature and at a dissolution pressure in the dissolution zone, whereby a first component of the gallium-containing feedstock is substantially completely dissolved and a second component of the gallium-containing feedstock as well as the crystallization seed remain substantially undissolved so that an undersaturated solution with respect to gallium-containing nitride is obtained;
(iv) subsequently setting the conditions in the crystallization zone at a second temperature and at a second pressure so that over-saturation with respect to gallium-containing nitride is obtained and crystallization of gallium-containing nitride occurs on the at least one crystallization seed and simultaneously setting the conditions in the dissolution zone at a first temperature and at a first pressure so that the second component of the gallium-containing feedstock is dissolved;
wherein the second temperature is higher than the first temperature.
A gallium-containing nitride crystal obtainable by one of these processes is also described. Further subject matter of the invention are a gallium-containing nitride crystal having a surface area of more than 2 cm2 and having a dislocation density of less than 106/cm2 and a gallium-containing nitride crystal having a thickness of at least 200 xcexcm and a full width at half maximum (FWHM) of X-ray rocking curve from (0002) plane of 50 arcsec or less.
The invention also provides an apparatus for obtaining a gallium-containing nitride crystal comprising an autoclave (1) having an internal space and comprising at least one device (4, 5, 6) for heating the autoclave to at least two zones having different temperatures, wherein the autoclave comprises a device which separates the internal space into a dissolution zone (13) and a crystallization zone (14).
In a yet another embodiment, a process for preparing a bulk monocrystalline gallium-containing nitride in an autoclave is disclosed, which comprises the steps of providing a supercritical ammonia solution containing gallium-containing nitride with ions of alkali metals, and recrystallizing said gallium-containing nitride selectively on a crystallization seed from said supercritical ammonia solution by means of the negative temperature coefficient of solubility and/or by means of the positive pressure coefficient of solubility.
A process for controlling recrystallization of a gallium-containing nitride in a supercritical ammonia solution which comprises steps of providing a supercritical ammonia solution containing a gallium-containing nitride as a gallium complex with ions of alkali metal and NH3 solvent in an autoclave and decreasing the solubility of said gallium-containing nitride in the supercritical ammonia solution at a temperature less than that of dissolving gallium-containing nitride crystal and/or at a pressure higher than that of dissolving gallium-containing nitride crystal is also disclosed.